Sprayed, closed-cell foam roofs originated in the 1950's in the wake of the development of the foams after World War II. The foam is typically made of two components which, when mixed together, give off an exothermic reaction. This causes the mixture to expand, trapping Freon in the cells and forming a material with insulating properties superior to any other commercially available insulation. (See RSI, June 1982, pp. 44-48).
The sprayed-in-place feature allows a contractor to provide an essentially seamless roof. Typically, the foam layer is sprayed onto the roof deck, coated with a waterproof layer and then covered with a layer of gravel. See for instance, the North Carolina Foam Industries, Inc. information sheet on Graveledfoam.TM. Roof Systems.
Numerous types of water impermeable coatings have been utilized throughout the years in an effort to prevent the sprayed foam layer from becoming water-logged, thus, resulting in leaks or other roof failure. Moore and Kashiwagi, (30th Annual Polyurethane Technical/Marketing Conference, Oct. 15-17, 1986, pp. 93-98), lists coatings of silicone, silicone and acrylic, asphalt and urethane. The roofing industry's position has always been that the foam is permeable to water and thus, it is necessary to protect it with a water-impermeable coating.
Ultraviolet degradation, heavy traffic, or movement of equipment across foam roofing will cause damage to the foam surface in the form of open or broken cells. Water will infiltrate into such cells. A water-impermeable coating prevents infiltration of water into such cells providing that the coating is totally intact and perfectly adhered. Any break in the coating would allow water to enter and flow laterally between the coating and the damaged foam surface. The ultimate result is that the foam becomes water-logged and the roof fails.
Additionally, there is a vapor drive from the inside of a building to the outside. When the roof deck has the insulating foam on top of it and the waterproof coating on top of the foam, the water vapor diffuses through the foam and condenses on the underside of the coating. Thus, a potentially destructive layer of water forms between the foam and its coating. Again, the ultimate result is that the foam becomes water-logged and the roof fails.